1. Technical Field of the Invention
The present invention relates to a fixing device including a heating roller, a pressurizing roller to be pressed against the heating roller, a belt wound around the outer periphery of the pressurizing roller and movable while it is held by and between the pressuring roller and heating roller, and a belt extension member for extending the belt thereon, thereby fixing an unfixed toner image formed on a transfer member, and an image forming apparatus incorporating such fixing device therein.
2. Description of the Related Art
Conventionally, in an image forming apparatus such as a facsimile, there is developed a so called heating roller type fixing device for contact heating and fixing an unfixed image formed on record paper, especially, a belt nip type fixing device which is composed of a heating roller and a belt pressure contact unit.
Now, description will be given below of a conventional belt nip type fixing device with reference to FIG. 1. In FIG. 1, a heating roller 100 is structured such that a hollow roller 101 is covered with HTV silicone rubber as a base layer 102 thereof and silicone RTV rubber is dip coated on the surface thereof as a top coat layer 103, whereby the surface of the heating roller 100 is finished in a state substantially similar to a mirror surface state. In the interior of the heating roller 100, as a heating source, there is disposed a halogen lamp 104. An endless belt 105 made of polyimide resin is extended with a given tensile force on and by three rollers each made of stainless steel, that is, a pressurizing roller 116, a support roller 117 and a support roller 118. The pressurizing roller 116 is energized toward the center of the heating roller 100 by a compression coil spring 109 serving as pressurizing means to thereby press the endless belt 105 against the heating roller 100. The pressurizing roller 116 can driven and rotated by a motor 110, whereas the support rollers 117 and 118 can be rotated following the pressurizing roller 116. A pressure auxiliary roller 111 is disposed on the inside of the portion of the endless belt 105 existing between the pressurizing roller 116 and support roller 117, and a pressing force is applied to the endless belt 105 using the pressure auxiliary roller 111, thereby controlling the tensile force of the endless belt 105 (See, for example, Japanese Patent 3084692, hereinafter referred to as JP'690).
In the fixing device disclosed in the above-mentioned JP'690, since the pressure auxiliary roller for controlling the tensile force of the endless belt is arranged on the inside of the endless belt, the space inside the endless belt must be formed wide. Also, because the positions of the pressurizing roller and support rollers are fixed, the amount of winding of the endless belt around the heating roller cannot be varied.
Therefore, it is necessary to increase the number of support rollers on which the endless belt is provided and, as a result of this, there arises a problem that the peripheral length of the endless belt must be increased. The increased number of support rollers and the increased peripheral length of the endless belt give rise to an increase in the size of the fixing device. Also, because of the increased peripheral length of the endless belt, when the endless belt heated in a nip portion thereof existing between the heating roller and itself moves along a given path, the temperature of the endless belt decreases due to the long peripheral length thereof. This results in an increase in a warming time which is necessary for the temperature of the fixing device to reach a desired temperature after the power supply is turned on, that is, a fixing executable temperature. Further, it is has been also found a fact that, besides the tensile force of the endless belt, the amount of winding of the endless belt on the heating roller provides a great element of the desired fixing performance of the fixing device according to various kinds of paper.